ATI v NVidia is a bit of a contentious subject! I think the general consensus from what I've read of other ppl is that ATI offer slightly better single-card solutions and NVidia win out for dual GPUs. However, at the low to mid-range gpu market I think NVidia still holds a definite lead, so depending on how much you wanted to spend, the 7600GS is very cheap and performs a shade better than your 6600gt and will allow you to stick another one in at a later date. However, imo its not worth sli'ing 7600's - you might as well see if you can stretch to a 7900gt as they are by far the best value cards around at the mo, and will more than double your graphics power in a single card.